1. Field of Invention
The present invention relates to a heterojunction bipolar transistor. More particularly, the present invention relates to a GaN heterojunction bipolar transistor with a p-type strained InGaN base layer and the fabricating method thereof.
2. Description of Related Art
In the fabricating technology of gallium nitride (GaN) npn-type heterojunction bipolar transistor, the key problem lies in the high resistance p-type base metal contact, and the reason thereof will be explained below in the following cross-sectional diagrams of the fabrication flow of a heterojunction bipolar transistor.
FIGS. 1A and 1B are cross-sectional diagrams illustrating parts of the fabrication flow of a conventional GaN heterojunction bipolar transistor.
First, referring to FIG. 1A, the first step of fabricating a GaN heterojunction bipolar transistor is to form a highly doped collector contact layer 102, a low doped collector layer 104, a p-type base layer 106, an emitter layer 110, and a highly doped emitter contact layer 112 on a substrate 100 in sequence. The dopant activation concentration of the p-type base layer 106 is about 1017cm−3.
Then, referring to FIG. 1B, dry etching processes are performed to the emitter layer 110 and the emitter contact layer 112 until a portion of the p-type base layer 106 is exposed, so as to form an emitter layer 110a and an emitter contact layer 112a. It is remarkable that the damages to the surface of the p-type base layer 106 during the dry etching process further aggravates the base contact resistance because the dopant activation concentration of the p-type base layer 106 is not very high (about 1017cm−3). Accordingly, it is more difficult to form the base ohmic contact, and further degrade the device characteristics.
There are two common solutions to the problem described above. The first solution is to optimize the conditions of the dry etching process. Even this method can reduce the surface related damages of the GaN layer of the p-type base layer after the etching process, however, base ohmic contact still can not be formed (this should be understood by those with ordinary skill in the art, so the details will not be described here) because of the increase of the gallium/nitrogen ratio on the surface caused, the surface roughness, contamination, and damages resulted from the dry etching. The second solution is to re-grow the GaN-based layer after the etching process. However, this method is complicated in process even though it reduces the increase of the base resistance.
In the aforementioned fabricating technologies of a conventional GaN heterojunction bipolar transistor, the device characteristics are poor because the surface of the p-type base layer is damaged during the dry etching process and the dopant activation concentration is too low.